1. Field of the Invention
The present invention relates to a particulate ferromagnetic chromium dioxide composition in which the particles are treated so as to render them particularly suited for use in magnetic recording.
Chromium dioxide finds application in various fields of magnetic recording, such as, for instance, in magnetic tapes for audio and video recordings, in tapes and memories for computers, in magnetic discs and cards, etc.
The ferromagnetic characteristics that make chromium dioxide particularly suited for such uses are mainly related to the high coercive force (Hc) which this material offers when it is obtained in an acicular form with a high length to width axial ratio, to its high saturation magnetization value (.sigma..sub.s) and its residual magnetization (.sigma. .sub.r). In addition, the facility for orientation which the particles exhibit when they are spread on a magnetic tape enables one to obtain hysteresis cycles of a configuration much closer to rectangular than would be possible when using conventional ferromagnetic compounds (e.g., .gamma.Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4).
2. Description of the Prior Art
According to the prior art, ferromagnetic chromium dioxide of the above indicated desired characteristics has been obtained according to several different processes. In some processes it has been obtained in an unmodified form, while in other processes it is admixed with suitable modifiers, some of which improve the granulometry or particle size and distribution of the product, others of which vary its ferromagnetic characteristics, such as the coercive force or the Curie temperature.
Processes for preparing pure, unmodified chromuim dioxide exhibiting high ferromagnetic properties are described, for instance in Italian Pat. No. 894,564 and in Italian Pat. No. 922,283.
Processes for preparing modified chromium dioxide having equally good ferromagnetic characteristics, are described, for instance, in Italian Patent No 606,749 to duPont and in Italian Patent application No. 23466 A/72, as well as in said U.S. application Ser. No. 350,033, filed on or about Apr. 11, 1973, by Ugo Montiglio, Pierfrancesco Aspes, and Giampiero Basile, entitled "Modified Chromium Dioxide Composition and Method of Preparing the Same".
However, it has been observed that the CrO.sub.2 displays a certain reactivity towards water and organic substances, particularly towards those containing easily oxidizable functional groups such as, for instance, hydroxy or amino groups.
These functional groups are often present in the resins used in the formulations for magnetic tapes and, upon their oxidation by the CrO.sub.2, it (the CrO.sub.2) is in turn reduced, at least on the surface, to chromium compounds of a lower valence, in particular, to Cr.sub.2 O.sub.3, which Cr.sub.2 O.sub.3 is not ferromagnetic.
From the foregoing it will be seen that if CrO.sub.2 is incorporated into a magnetic tape and this is subjected to recording, after a certain period of time there will be observed a drop in the output level, which drop is related to the lowering of the residual magnetization of the ferromagnetic pigment.
One method for overcoming this drawback is described in Italian Patent No. 865,839 to duPont, and involves reducing the surface of the chromium dioxide particles by means of suitable reducing agents, such as H.sub.2 S, alkaline bisulfites, and alcohols having from 1 to 18 carbon atoms.
The thus treated chromium dioxide, which under X-ray diffraction examination shows a line corresponding to an interplanar distance of 3.151.+-. 0.005 A, is much less reactive toward water and organic substances containing easily oxidizable functional groups.
This reducing treatment has, however, a serious drawback, namely, it tends to lead to a very pronounced deterioration of the magnetic characteristics of the CrO.sub.2, so that the value of residual magnetization drops to about 30% of that of the untreated product.